A gas laser device uses a special laser gas supplied from a laser gas supply unit and exhausts a laser gas degraded by an electric discharge through a gas exhaust unit, and accordingly, a gas cylinder in the laser gas supply unit must be replaced at predetermined intervals. At this time, a laser gas is introduced from the external laser gas supply unit to fill a laser oscillator with a predetermined amount of the laser gas after the interior of the laser oscillator has been exhausted by the gas exhaust unit. In general, the sequence thereof is such that a valve disposed in the vicinity of the laser oscillator is closed when the gas cylinder is replaced, and is opened when the laser gas is introduced.
However, when the empty gas cylinder is to be replaced with a full one, air enters a pipe disposed between the laser gas supply unit and the valve on the laser oscillator side, and if the gas laser device is operated in this state, a laser gas containing air is introduced into the laser oscillator, and thus a problem arises in that the laser output is reduced and becomes unstable.